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(in-fek′shŏn ) [L. infectio, discoloration, dye] A disease caused by microorganisms, esp. those that release toxins or invade body tissues. Worldwide, infectious diseases such as malaria, tuberculosis, hepatitis viruses, and diarrheal illnesses produce more disability and death than any other cause. Infection differs from colonization of the body by microorganisms in that during colonization, microbes reside harmlessly in the body or perform useful functions for it, e.g., bacteria in the gut that produce vitamin K. By contrast, infectious illnesses typically cause bodily harm.

ETIOLOGYThe most common pathogenic organisms are bacteria (including mycobacteria, Escherichia coli, mycoplasmas, spirochetes, chlamydiae, and rickettsiae), viruses, fungi, protozoa, and helminths. Life-threatening infectious disease usually occurs when immunity is weak or suppressed (as during the first few months of life; in older or malnourished persons; in trauma or burn victims; in leukopenic patients; and in those with chronic illnesses such as diabetes mellitus, renal failure, cancer, asplenia, alcoholism, or heart, lung, or liver disease). Many disease-causing agents, however, may afflict vigorous people, young or old, fit or weak. Some examples include sexually transmitted illnesses (such as herpes simplex or chlamydiosis), respiratory illnesses (influenza or varicella), and food or waterborne pathogens (cholera, schistosomiasis).

TRANSMISSIONPathogens can be transmitted to their hosts by many mechanisms: inhalation, ingestion, injection or the bite of a vector, direct (skin-to-skin) contact, contact with blood or body fluids, fetomaternal contact, contact with contaminated articles (fomites), or self-inoculation.

In health care settings, infections are often transmitted to patients by the hands of professional staff or other employees. Hand hygiene before and after patient contact prevents many of these infections.

DEFENSESThe body's defenses against infection begin with mechanisms that block entry of the organism into the skin or the respiratory, gastrointestinal, or genitourinary tract. These defenses include chemicals, e.g., lysozymes in tears, fatty acids in skin, gastric acid, and pancreatic enzymes in the bowel; mucus that traps the organism; clusters of antibody-producing B lymphocytes, e.g., tonsils, Peyer patches; and bacteria and fungi (normal flora) on the skin and mucosal surfaces that destroy more dangerous organisms. In patients receiving immunosuppressive drug therapy, the normal flora can become the source of opportunistic infections. Also, one organism can impair external defenses and permit another to enter; e.g., viruses can enhance bacterial invasion by damaging respiratory tract mucosa.

The body's second line of defense is inflammation, the nonspecific immune response. The third major defensive system, the specific immune response, depends on lymphocyte activation, during which B and T cells recognize specific antigenic markers on the organism. B cells produce immunoglobulins (antibodies), and T cells orchestrate a multifaceted attack by cytotoxic cells. SEE: B cell; SEE: T cell; SEE: inflammation for table

SPREADOnce pathogens have crossed cutaneous or mucosal barriers and gained entry into internal tissues, they may spread quickly along membranes such as the meninges, pleura, or peritoneum. Some pathogens produce enzymes that damage cell membranes, enabling them to move rapidly from cell to cell. Others enter the lymphatic channels; if they can overcome white blood cell defenses in the lymph nodes, they move into the bloodstream to multiply at other sites. This is frequently seen with pyogenic organisms, which create abscesses far from the initial entry site. Viruses or rickettsiae, which reproduce only inside cells, travel in the blood to cause systemic infections; viruses that damage a fetus during pregnancy (such as rubella and cytomegalovirus) travel via the blood.

DIAGNOSISAlthough many infections (such as those that cause characteristic rashes) are diagnosed clinically, definitive identification of infection usually occurs in the laboratory. Carefully collected and cultured specimens of blood, urine, stool, sputum, or other body fluids are used to identify pathogens and their susceptibilities to treatment.

TREATMENTMany infections, like the common cold, are self-limited and require no specific treatment. Understanding this concept is crucial because the misuse of antibiotics does not help the affected patient and may damage society by fostering antimicrobial resistance, e.g., in microorganisms such as methicillin-resistant Staphylococcus aureus. Many common infections, such as urinary tract infections or impetigo, respond well to antimicrobial drugs. Others, like abscesses, may require incision and drainage.